Two design objectives of a microwave oven are (1) that the energy distribution within the cavity be such as to provide uniform heating in food and (2) that there be an acceptable load impedance on the magnetron with any of a variety of food loads in the cavity. With regard to the second objective, an acceptable load impedance is one which will provide sufficient loading for the magnetron to prevent excessive anode heating without loading the magnetron so heavily that it will fail to oscillate at the correct frequency and shift to another mode. In other words, the magnetron should be coupled tightly enough so as to get good efficiency or maximum power output but loosely enough to give good frequency stability. The magnetron performance effects of impedance matches are well knwon and generally specified by magnetron manufacturers on Reike Diagrams.
When microwave ovens were first introduced for food cooking and industrial processing, some models had the output probe of the magnetron inserted directly into the microwave enclosure. It was found that some improvement could be gained in heating uniformity by positioning a moving device commonly referred to as a mode stirrer in the enclosure. However, with the direct insertion configuration, little was done to provide the magnetron with an acceptable impedance load with a variety of food loads. Accordingly, it was common to have the magnetron operating inefficiently and/or with poor frequency stability.
One way of providing an acceptable impedance match for the magnetron is to couple it into a waveguide; this has become the conventional microwave feed system. Typically, the output probe of the magnetron is inserted into a waveguide approximately one-quarter wavelength from a shorted end so that substantially all the microwave energy couples in the opposite direction. Generally, the end opposite the shorted end opens into the microwave enclosure. A mode stirrer means is commonly positioned in the waveguide or adjacent to it within the microwave enclosure. Coupling the magnetron output probe into a waveguide and the waveguide into the cavity provided for smaller impedance variations on the magnetron as a result of different food loads.
The use of a waveguide external to the microwave cavity has several significant disadvantages. First, there is the cost of the waveguide that obviously must be included in the price of the oven. Second, there are microwave energy losses in the waveguide which reduce the efficiency of the system. Third, the coupling of microwave energy into the cavity from a waveguide to set up standing waves which are varied by a mode stirrer has not produced the most desirable uniformity in cooking.
The elimination of the external waveguide creates many significant problems. For example, an acceptable impedance match must be provided for the magnetron for a variety of food loads. Also, uniformity of heating within the foods must be provided. Furthermore, if the microwave feed system is used in a combination oven which has an additional heat source for self-cleaning by pyrolysis, a means for isolating the magnetron from the self-cleaning temperatures must be provided. Also, there must be a method for sealing the feed system to prevent leakage of microwave energy.